Regional Karst Geology - West Coast

Mary Trayes

On the West Coast of the South Island all karst areas lie west of the Alpine Fault and apart from one small area of Paleozoic limestone (late Cambrian - Ordovician Mt Arthur Group), which has been metamorphosed to marble, this karst is formed from mid Tertiary aged limestones. A small area of fossiliferous Paleozoic Limestone near Reefton (Devonian Reefton Group) is significant to New Zealand's geological history, but not karst forming.

The Alpine Fault is a major New Zealand geological feature which marks the convergence of two active continental plates, the Australasian Plate and the Pacific Plate. Convergence at the boundary between these two plates began around 50 million years ago and continues to this day, with movement on the fault being strike-slip, i.e. there is both vertical uplift (forming today's Southern Alps) and horizontal slip (northward). Scientists estimate that the next Alpine Fault earthquake (probability high in our lifetime) will generate sufficient energy (Force 8+ on the Richter Scale) to cause 1 - 3m of uplift and 7 - 8m of horizontal movement.

Over the Tertiary period this strike-slip movement has separated large blocks of rock which used to be continuous across the fault. The Red Hills ultramafic area in South Westland and the allied Dun Mountain ultramafic area near Nelson are now 480 kilometres apart while the marbles of Mt Arthur are linked to the many marble remnants found throughout Fiordland.

During the Oligocene - Miocene period most of New Zealand was under the sea and large beds of carbonate sediments were laid down, blanketing older sediments and Paleozoic basement rocks. The carbonate sediments eventually became limestones and mudstones which have been since exposed due to upthrust along the Alpine Fault. This mountain building phase, the "Kaikoura Orogeny" began some 20 million years ago and continues down to the present day.

Currently the mountains in and around Mt Cook are "growing" at 10mm per year. But they are also eroding at much the same rate so that on the coastal shelf there are large depths of recently eroded sediments, plus earlier glacial gravels, overlying large areas of carbonate rocks. As the non-marine coal-bearing sediments under the carbonate rocks have the potential to contain gas and oil, much of the coastal shelf off the West Coast was explored by petroleum companies in the 1960-70s using seismic reflecting methods. The limestones in particular are good seismic reflectors and have contributed greatly to our current understanding of West Coast bathymetry.

The 600 kilometre coastal strip from Kahurangi Point in the north to Big Bay in the south, otherwise known as the "West Coast", is underpinned by some of New Zealand's oldest rocks. These basement rocks are Paleozoic remnants from proto-New Zealand, when this country, along with eastern Australia and Antarctica, formed the southwest segment of Gondwanaland. They are formed from either late Cambrian - Ordovician gneiss (Victoria Paragneiss, Pecksniff Sedimentary Gneiss, Charleston Gneiss) or from a sandstone - shale formation of the same period, commonly known as greywhacke (Greenland Group Rocks) or from a late Devonian - early Carboniferous biotite granitoid rock (the Karamea Granites) which was intruded over a wide area. Extensive outcrops of basement rock, from which overlying sediments, including limestones, have been eroded, still exist as the granite - gneiss Paparoa and Victoria Ranges, whilst small exposures of all three types can be found along the coast.

The only Paleozoic karst found on the West Coast is a small area at Springs Junction. Here the Sluice Box Limestone formation forms the southern most known outlier of the Mt Arthur Group which includes marble. The "Sluicebox," is a short gorge lying immediately west of the Alpine Fault which has been carved through the marble by the Maruia River. The well decorated Guillotine (or Hunters) Cave can be found in this formation a short distance from Springs Junction. In some places mid-Tertiary limestones lie unconformably over basement rocks - for instance pockets of Oligocene age Takaka Limestone in Kahurangi National Park rest on an ancient peneplain formed of Karamea Granites - but in most places there are other Mesozoic - early Tertiary sediments in between. These include breccias (mid Cretaceous Hawkes Crag Breccia), coal measures (late Cretaceous Paparoa Coal Measures and Eocene Brunner Coal Measures), sandstones (Eocene Island Sandstone:) and mudstones (early Oligocene Kaiata Mudstone). The Oligocene Limestones are in turn topped in many places by various Miocene formations such as the Welsh Formation (calcareous mudstones and limestones) and Stillwater Mudstone (often known as Blue Bottom or simply "papa").

The final cap which lies over much of the lowland West Coast landscape, is a layer of Pliocene non-marine gravels (Old Man Formation) and Quaternary glacial gravels. In many places relatively recent Tectonic activity has uplifted the beaches to form raised beaches or terraces behind the present coastline. In some places gravels on these terraces lies directly over limestone, e.g. at Darkies Terrace, Point Elizabeth. Sluicing these gravels for gold was difficult due to the propensity for water to drain away underground from dams and water-races.

With the exception of a very small area of strongly foramineral late Paleocene limestone in South Westland (the Abbey Limestone Formation at Abbey Rocks, south of the Paringa River), the remaining limestones of the West Coast region have been formed during the Oligocene period between 37 and 25 million years ago. They are all considered to be sufficiently similar in age and structure to be placed in the one group, the Nile Group. Outcrops of Nile Group limestones increase from south to north as the gap between the coast and the Alpine Fault widens, i.e. there is far more limestone and consequently more karst and caves north of the Taramakau River than south. In fact there is a large gap to the south synonymous with the "Beech Gap" (Taramakau River to Paringa River) where there are only two minor outcrops. Further south limestone is only found at the coast and on offshore islands.

North of the Taramakau the best karst and caves are formed in Potikohua Limestone which is found, in varying thicknesses, from Fox River to the Punakaiki River. It overlies a less calcareous, more sandy formation, the Tiropahi Limestone. Potikohua Limestone is formed from a hard white flaggy polyzoan biosparite which averages 90% CaCO3. Most of the karst and caves formed in Potikohua Limestone are now protected within Paparoa National Park .

All the other formations are more variable, often with muddier bands grading down to actual mudstone, and of variable thickness. The Tarapuhi Limestone Member, which forms the Point Elizabeth area, is the best ranked (highest CaCO3) of the Cobden Limestone Formation while well decorated caves can be found in Takaka Limestone (Megamania), Stony Creek Limestone (Honeycomb) and the Jackson Formation (Serendipity). Caves have also formed in locally highly calcareous Island Sandstone near Rapahoe (Cavern Cave).

The temperate and humid West Coast climate with a mean annual temperature of 12°C and mean rainfall at the coast of 2500mm (and more inland in many places) is conducive to relatively rapid karst formation in the Nile Group limestones. Most karst areas are heavily vegetated with either mixed podocarp - beech forest or coastal forest. This makes prospecting for new caves and trips to known ones off the beaten track, hard work. Given that since 1989 when the last ACKMA Conference was held on the West Coast, at least three major new caves have been found (Megamania, Abyssinia and Te Ana Puta), there are probably many more out there waiting to be found.

Maps

No one geological map adequately shows all the karst and cave areas of the West Coast Region. The best is the 1:100,000 Geological Map of the South Island, however the scale is necessarily small. Larger scale maps are available but no one complete series covers all the West Coast. Therefore the maps references for the following work vary from one area to another. The old DSIR 1:250,000 maps are being superseded gradually by the new Q Series maps at the same scale but 1:63,630 (inch to the mile) maps are all that is available for much of the West Coast. An exception is the 1:50,000 Kumara - Moana Geological Map which details the Cobden Limestone very well. A selection of geological maps will be available at the conference.

Map (showing most of the karst within Paparoa National Park)

Taken from p.11, The Hydrological Behaviour of a High Flooding Frequency Karst System in New Zealand courtesy Scott Crawford, Dept. of Geography, University of Auckland, 1987.

Some significant limestone formations in the Nile Group are: (north to south)

Area	Formation	Example Karst Feature/s
Heaphy	Takaka Limestone	Megamania Cave, Heaphy Bluff
Oparara	Stony Creek Limestone	Honeycomb Hill Cave, Fenian Caves
Murchison	Matiri Formation	100 and 1000 Acre Plateaus
Inangahua Junction	Whitecliffs Formation	Profanity Cave, Damnation Cave
Cape Foulwind	Waitakere Limestone	Rock quarry for cement making
Charleston to Punakaiki	Potikohua Limestone	Metro Cave, Te Tahi Cave Fox River Cave, Babylon Cave Pancake Rocks, Xanadu Cave system
Greymouth	Cobden Limestone	Grey River Gorge, Te Ana Puta Cave
Jackson's Bay	Jackson Formation	Serendipity Cave, Jackson Head

NB: A more comprehensive list of West Coast cave and karst areas is available on the database, Calcareous Rocks West of the Alpine Fault, New Zealand by M Trayes, 2005. [opens as MIcrosoft Excel spreadsheet]